At present, many integrated circuits include RC oscillators configured to generate and offer internal clock signals to be used in each circuit. For example, a multivibrator for generating rectangle clock signals is used, and a clock frequency depends on an RC time constant of the multivibrator. However, due to production tolerance, for accurate operation of the oscillator, an expected clock frequency is corrected or calibrated generally.
In order to achieve this purpose, external clock signals are offered to a calibrating circuit generally, and generated by means of a crystal oscillator or an external microcontroller. Calibration coefficients of the RC oscillator are determined according to the external clock signals. These calibration coefficients may be stored in a nonvolatile memory during production of the integrated circuit with the RC oscillator so that a calibration processing is to be executed once. However, the nonvolatile memory (for example, one-time programmable (OTP) memory) occupies an area on the integrated circuit, thereby increasing a production cost.
In another traditional method for calibrating an internal RC oscillator, a crystal oscillator or a microcontroller is further provided to the integrated circuit to offer external reference clock signals, so that calibration should be implemented when electrifying the integrated circuit each time. Under this circumstance, calibration coefficients are stored in a volatile memory. But an external component (such as the crystal oscillator or the microcontroller) may generate additional cost. In addition, operations of the crystal oscillator or the microcontroller consume power, which is especially not expected in a mobile device.
An interface chip may also be equipped with one crystal oscillator to accordingly acquire a high precision. However, the interface chip needs two more pins to be led out, and a cost of an application system may be relatively high. Another method is to use a built-in RC oscillator on the interface chip, and this solution has low cost and would not increase the number of the pins, but the RC oscillator may not have favorable accuracy and stability. A traditional method for improving the accuracy of the RC oscillator is that the interface chip is detected before encapsulation during production, and R or C in a bare chip is subjected to fine tuning and correction with a laser photoetching method. In this method, the interface chips are subjected to a test during production, and corrected one by one according to a test result. Therefore, the water test has high cost, and this solution is unacceptable as for a chip of low value.
As to a problem in the related art that cost is great when improving a calibration accuracy of the RC oscillator, no effective solution has been proposed yet till now.